Hawaiian Island Dissolving From Within

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Plan your island getaway now: In time, the mountainous tropical
paradise of Oahu will erode, according to new research, with the
biggest losses coming from within the island itself.

To be accurate, you do have some time to book
that vacation before Hawaii's Oahu flattens from an island
into a
low-lying seamount. Researchers writing in the upcoming
February 15 issue of the journal Geochimica et Cosmochimica Acta
estimate that the volcanic island will continue to grow, thanks
to plate tectonics, for another 75,000 to 1.75 million years.
After that, however, the forces working to eat away at Oahu from
the inside out will begin to triumph.

Researchers at Brigham Young University in Utah investigated the
forces that add and subtract material from Oahu. The island
offers an ideal place to conduct such a study, the researchers
said, as it consists of one kind of rock that is exposed to very
different levels of precipitation. Various regions in Oahu can
record between 2 and 23 feet (0.6 to 7 meters) of precipitation a
year, depending on the local climate. [ Gallery:
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The researchers measured solids dissolved in both surface and
groundwater from 45 streams and 30 springs and wells around the
island, adding those new measurements to previously reported
data, for a total of 170 water samples scattered across Oahu.

Using that data, scientists calculated the
mass Oahu loses each year. Although one might expect rain to
carry away most of the soil in such a wet climate, underground
freshwater springs actually removed the bulk of the mineral
material from Oahu, the researchers found.

"More material is dissolving from those islands than what is
being carried off through erosion," study researcher Steve
Nelson, a Brigham Young University geologist, said in a
statement.

In fact, groundwater carried between three and 12 times as much
dissolved solids compared to surface water, the researchers
report.

Oahu is made up of the remnants of two collapsed shield
volcanoes, the kind known for
burping out thick, oozy lava that hardens into new land. One
volcano, Waianae, was active from about 4 to 2.6 million years
ago; the other, Koolau, developed later.

Today, Oahu grows not because of volcanism, but from geologic
uplift. As the younger Hawaiian Islands push the Pacific tectonic
plate downward, nearby Oahu "pops up," as if on a seesaw. That
uplift pushes Oahu's landforms upward at a rate of 0.2 feet (0.06
m) per thousand years, enough (for now) to compensate for the
losses caused by groundwater carrying away the island's mass.

Researchers hope that the same methods they used on Oahu can help
clarify how other tropical islands change in response to
different climate conditions.